Marilyn is a Trainer who believes only Pokémon she finds to be cute are worthwhile. Beyond that, she prefers things being cuter in their own right, calling Dawn and Brock "Dawny" (Japanese: ヒカリン Hikarin) and "Brocky" (Japanese: タケリン Takerin) respectively, because she believed they were cuter names.
She only believes that a Pokémon is cute if its height is no more than 2 feet, if it has not evolved, and most importantly if it looks "absolutely lovely", as she doesn't consider everything that meets the former two rules to be cute, including Mime Jr. and Piplup. Meowth also meets the height and evolutionary standards, although Marilyn didn't even seem to notice Meowth at any point.
After seeing Dawn's Buneary, she fawns over how cute it is (dismissing Piplup as not cute enough), and challenges Dawn to a battle. During the battle, she orders her Cherubi not to attack, in order to stall and keep the battling going so she could witness a "valiant battle". Brock gets irritated by Marilyn's superficial attitude towards Pokémon, and decides to teach her about the inner strength and beauty of Pokémon with a battle.
Marilyn claims her perception of "cute Pokémon" was altered to fit in more with normal convention, because she was teased as a child for believing the cutest Pokémon ever was Shellder, a sentiment none of her friends agreed with.